In many systems and devices, especially, for example, devices and systems which include high speed digital communication circuits, more than one signal may be present on a given signal line at the same time. In some cases, this may involve an incident signal on a signal line from an outset of a first device (e.g., a memory controller) to an input of a second device (e.g., a memory device), and a corresponding reflection signal on the same signal line from the input of the second device back to the output of the first device. In those cases, the ratio of the magnitude of the reflected signal to the magnitude of the incident signal may be referred to as the reflection coefficient of the input of the second device. In other cases, the first signal may be an output signal of a first device which is transmitted along the signal line to an input/output of a second device, and the second signal may be an output signal of a second device which is transmitted along the signal line to the input/output of a second device. This may be the case when the first device and second device engage in full-duplex communication via the signal line.
The measurement and analysis of such signals may be difficult and complicated.
Prior solutions in the case of measuring incident and reflected signals have included cutting the signal line, attaching a radio frequency (RF) connector to the cut signal line, and measuring the reflection coefficient as the S11 parameter using a network analyzer. This is clearly undesirable. Furthermore, the time constant of the network analyzer depends on the IF bandwidth, which may typically be about 100 kHz (which means a time constant of about 10 μs.). However, in some cases such as a double data rate (DDR) memory device, the data rate along the signal line in actual operation may be in the range of 2 Gbps, which has a fundamental frequency component at 1 GHz (1 nanosecond) and a third harmonic of 333 ps. This means that the network analyzer cannot measure the reflection coefficient for the device at its normal operating speed.
Other solutions for some types of RF signals have included embedding one or more directional couplers in the signal line so that coupled signals may be measured in both directions. Adding directional couplers to a signal line simply to allow measurement of reflection coefficients involves an additional cost, additional size, and additional signal loss, all of which are undesirable. And in a circuit with many signal lines and device inputs or outputs to be measured, these effects are all multiplied. Furthermore, in the case of digital signals, such directional couplers are not typically employed.
Prior solutions in the case of measuring two output signals of two devices on a shared signal line involve stopping transmission by one of the two devices to make one of the signals silent while measuring or evaluating the other signal. However, it is often desired to test a circuit in its actual operating mode where two devices are transmitting at the same time such that two signals are present on the signal line at the same time. Accordingly, for such tests, it is not possible to stop transmission from one of the devices while measuring a signal from the other device.
Accordingly, it would be desirable to provide a new system and method for measuring two signals which are simultaneously present on a signal line. It would further be desirable to provide a new system and method for measuring the reflection coefficient of an input or output of a first device connected to second device via a signal line.